text:
Why did narrowbody jetliners take so long to switch over to high-bypass engines?
High-bypass turbofans were introduced into the airliner market around 1970, when the first widebody jetliners were released (the 747 was first, followed in rapid succession by the DC-10, L-1011, and A300); however, narrowbodies continued to rely on low-bypass turbofans (primarily the Pratt & Whitney JT8D) for considerably longer.  To the best of my knowledge, the first narrowbody powered by high-bypass engines was the DC-8-70 (a re-engining of the DC-8-60), which entered service in 1982; the first narrowbody built with high-bypass turbofans, the 757, did not enter service until 1983.  As for the other narrowbodies of the time:

707, 720, 1-11, F28: No high-bypass version ever became available (although high-bypass versions of the 707 and 1-11 were proposed, they never entered production).
727: The only 727s that ever flew with high-bypass engines were four 727-100s reengined and renamed 727-100QFs in 1992.
737: The first high-bypass version, the 737-300, was only introduced in 1984.
DC-9: Stubbornly held onto the JT8D into the mid-1990s, only switching to high-bypass engines with the DC-9-90.

What kept the narrowbodies from immediately, or almost immediately, taking advantage of the greater power and lesser fuel consumption and noise of high-bypass turbofans?

switching engines, in general, isn't easy. finding a proper engine with proper thrust and weight isn't easy either. it simply takes time for CFM56 to emerge and engineered to be a usable replacement for JT8D.

There was no suitable high-bypass-ratio turbofan engine available.
What were the narrow bodies flying in the 1960s and 70s? Here is a list:

Sud Aviation Caravelle
Boeing 707
Douglas DC-8
Hawker-Siddeley Trident
BAC One-Eleven
Boeing 727
Tupolev 134
Tupolev 154
Douglas DC-9
Vickers VC-10
Ilyushin Il-62
Boeing 737

The thrust needed for those aircraft, about 10 tons per engine, was not covered by the first generation of high-bypass-ratio engines. Remember, their development was started with the Heavy Logistics System (CX-HLS) program which eventually resulted in the General Electric TF-39 and the C-5 Galaxy. The maximum thrust of the TF-39 (43,300 lbf or 193 kN) is twice that of the Pratt & Whitney JT8D (21,000 lbf or 93.4 kN) which powered the Boeing 707, 727 and 737, the DC-8 and -9 and the Mercure. Another result of the CX program was the Pratt & Whitney JT9D which was in the same thrust class as the TF-39 and its civilian spin-off CF6.
Other early engines with higher bypass ratio like the General Electric CF700 were much too small with 4,200 lbf or 18.68 kN static thrust and did not offer enough of a jump in bypass ratio to become interesting for airliners.
It was not before the CFM56 became available in the 1980s that a suitable high-bypass-ratio engine in the 10-ton class existed. One reason for the long development time of the CFM56 was the delay in an export license for the engine core technology which was derived from the military F101 design. And when the CFM56 entered the market, it was first used to re-engine old Boeing KC-135 (the engines for the French KC-135s were the first big order), 707 and DC-8 aircraft which benefited much more from the new engine than the smaller mid- and short-range airliners.
The competition to the CFM56 (IAE V2500 and Pratt & Whitney 6000 series) both entered the market much later. There simply was no suitable engine available any sooner!

The CFM56 didn't take that long to trickle down to the smaller narrowbodies; the French government chose it to reengine its KC-135s in 1978, United Airlines decided to upgrade its DC-8s with the new engine in 1979, and Boeing released the design for the 737-300 in mid-1980, just six months after the engine won the USAF KC-135 contract.

@Sean: Yes, but it was still more than one decade later than the first widebodies with high bypass turbofans. The question asks why it took so long and this is the answer: There was nothing available before.

@PeterKämpf In general it's easier to make a smaller engine than a big one, so if there was demand for a small high-bypass engine it would come first. For example, JT9D had fan case distortion issues which is directly related to such a large fan. Another example today is geared turbofans: it took 45 years to advance from bizjet engines to 30k thrust, and they haven't shown up anywhere greater.

@user71659 You are right about engine size. The big difference, however, was that the US military funded only 20 ton class turbofans and left the 10 ton class to the civilian sector. And the Snecma-GE cooperation had to use a military-funded hot section (from the F101) to build their CFM56. Civilian funds were simply not available earlier, even though the smaller engine would pose less risk.

Problem with this argument: the CF34 (a smaller military-derivative high-bypass turbofan) was available in the early 1970s and was (and still is) just about the perfect size to power the smaller narrowbodies.

@Vikki The CF34 is a civilian version of the military-funded TF34 and a bit too small for the 10-ton class of engines which were needed for narrowbodies. So the argument holds that civilian funds were not committed, although demand was clearly there.

@PeterKämpf: It's plenty large enough to power a regional jet (as indeed it would do later on).

@Vikki Regional jets were not a thing until the late Eighties. Back then, those were mostly turboprops. With plenty of bypass ratio.

@PeterKämpf: The DC-9-10 and Caravelle I-VI (of similar size to or smaller than many of the RJs that the CF34 would ultimately power) would like to disagree with you.

@PeterKämpf: So basically if the S-3'd been a bit heavier (and thus required GE to build the TF34 with 11k-12k lbf rather than 9.3 lbf), we could've had high-bypass narrowbodies in 1972 rather than 1982?

@Vikki Theoretically yes, but it took an oil shock in 1973 to make re-engining existing designs economically viable.

Early high-bypass engines were expensive and unreliable.
The effort in developing new technology and designs, plus the cost of building a larger fan add cost. Then all of the new technology is inherently less reliable than existing proven designs.
The JT9D was notoriously unreliable, causing major problems in the development of the 747. Development problems and spiraling costs from the RB211 bankrupted Rolls-Royce, had huge failure rates, nearly causing an accident (Eastern Airlines Flight 855), and ultimately killing the L1011. CF6 fan disk failures caused National Airlines 27, sucking a passenger out the window, and the fatal United Sioux City crash.
Even what we think of as the most reliable engine today, the CFM56, when new had a fan failure leading to a fatal crash (Kegsworth), combined with water ingestion issues that nearly lead to a crash (TACA 110).
When you have such compromised technology, you narrow it in the place that makes sense. High bypass engines brought high efficiency, reducing fuel burn. They also had more thrust, allowing heavier planes, carrying more fuel. Both advantages combine to enable long-haul direct flights which passengers like. These are flown with widebodies, so naturally these engines were used in widebodies.
Short-haul narrowbodies carry less fuel per flight, and can refuel frequently, so there wasn't an advantage here. Only as the compromises with high-bypass engines were solved, then did the technology trickle down.
You see a similar thing today with carbon fiber composite fuselages. They're expensive to build, expensive to fix, and there is uncertainty over long-term costs. But the weight savings opens up ultra-long-haul flights, hence the A350 and 787.

Note, too, that the high bypass engines all debuted on >2 engined aircraft, so if an engine failure occurred, you continued on 2/3 or 3/4 of engine power, not 1/2. Until the tech matured and reliability increased, that extra 1/6 to 1/4 of designed power made a difference.

@FreeMan: Given that the (twin-engined) A300 first flew in 1972, that would imply that high-bypass technology matured fairly rapidly.

@Sean I was going to note the A300 was twin engined, but if you look at its history, it really didn't sell well until about 10 years after everybody else, placing it more in the 767 era. (Famously Airbus was stuck with a bunch of white tails, unordered planes that were built) Then Airbus turned around and put out a 4 engined plane, the A340, at the point the market went to twins.

@user71659: The A300 didn't sell poorly because of unreliability; it sold poorly because ETOPS wasn't around yet.

@Sean That would only affect transatlantic flights. The A300 still was perfectly fine for the high-density short- to medium-haul it was designed for (the air bus missions). It ended up selling well doing just that around southeast Asia and Europe.

@user71659: Yes, where the short-to-medium-haul market was busy enough to support that.  The North American and European short-to-medium-haul markets didn't have enough demand to support the A300.

@Sean I suspect the hesitancy to buy the A300, despite it being technologically superior, was due to concerns about the long-term viablity of the brand new Airbus. Concorde was a commercial failure, the L-1011 made Lockheed quit, the Mercure didn't sell. A conservative decision would be to stick with Boeing and Douglas who weren't going anywhere.

No, reliability was not the problem. US airlines would only buy widebody aircraft that can be used on routes to and from Hawaii. The A300 furthermore was held back by a ground pressure limit that was designed to exclude it from the US.

I don't think Eastern Airlines Flight 855 can really be blamed on high-bypass engines; no engine is going to work well when you let all the oil out.

@SomeoneSomewhere The background was Eastern was experiencing a RB211 in flight shutdown rate from .25 to 1 per 1,000 EFH (ETOPS mandates 0.01 per 1,000, CFM56 is around 0.003), with as much as 18 engines failing in one month. This disastrous performance led to them mandating chip detector inspections daily which drastically increased the chance of errors (it was the 5th IFSD due to chip detector leaks), and led to the workers cutting corners in inspection and motoring of the engines.

One reason would be that high bypass engines tend to be larger, especially having a larger diameter.
This requires often quite significant redesign of the aircraft to fit them when the aircraft wasn't designed with them in mind from the outset.
The 737 for example required a longer undercarriage to allow the larger diameter engines to fit under the wing. Aircraft with tail mounted engines require almost certainly strengthening of the rear fuselage, especially the mounting pylons and their support structures, which adds weight, changes the center of gravity, etc. etc..
It probably just wasn't worth the cost until noise regulations made aircraft with turbojets harder to sell and operate.

The 737 didn't get a longer undercarriage until the MAX; the Classic and NG 737s have the same-length landing gear as the 737-100/200 (otherwise they wouldn't need their hamster-pouch engine intakes).


text:
Why aren't there any jets with horizontal-stabilizer-mounted engines?
Looking at the spectrum of where jet aircraft put their engines, there are examples of almost every place that could conceivably used to carry a jet engine:

slung under the wings
buried inside the wings
on top of the wings
on the sides of the aft fuselage
in the aft fuselage
on/in the vertical stabilizer

Notably absent, however, are jets with engines having anything to do with the horizontal stabilizer.
Why aren't there any jets with engines mounted on/in their horizontal stabilizer?

Related: Would there be any benefit in placing the aircraft engines on the elevator (end-tips)?

Because we want the stabilizer to have smooth air, hence away from anything, including the engine.

I think some fighter jets like the F-14 would come pretty close.  But in general, it's just a matter of structural strength: you'd have to make the stabilizer much stronger to handle the thrust of the engine.

Your linking game is en pointe. Especially the first example!

With the DC-10/MD-11, the engine isn't IN the vertical stabilizer.  Rather, the stabilizer is mounted on top of the engine.  We could probably find a lot of single-engine jets with the same basic design, e.g. https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter

@jamesqf: Hence the "on/" part of that example.  Besides, the long intake is mounted in the stabilizer, with essentially all the same aerodynamic effects that would result from the engine itself being in the stabilizer.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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text:
What flying boat has the highest maximum takeoff/landing sea state?
I assume that flying boats have limits on the sea states they’re certificated to take off and/or land in (for instance, hitting a ten-meter wave head-on in a five-meter wooden plane is probably somewhat hard on the plane’s structure, not to mention its contents), the aircraft’s precise maximum sea state rating presumably depending primarily on its size and structural strength.
Which flying boat can legally take off and/or land in the roughest seas?  Are there any flying boats certificated to operate in full hurricane conditions?

The Grumman Albatross was capable of open sea operations and could operate in as high as ten foot seas, possibly higher with JATO rockets installed.

The Albatross was capable of open-ocean landings (optimally in seas no
  higher than 10 feet) due to its deep-V hull cross-section and long
  keel length, although higher seas could be downright punishing on the
  aircraft and crew. HU-16s could utilize jet assisted takeoff (JATO)
  bottles to shorten takeoff runs in rougher seas.

This Albatross pilot offers some more specifics on the certifications and says

Its certified for class 6 seas, which can be swells up to 10 ft. or waves up to 6 ft.

Although he says he would not really want to do it as his operation has no need but has landed in 4ft seas.
According to this source a Boeing 314 Clipper was successfully landed in 30 ft. seas but thats by no means regular operations.

The weather was far from ideal, with a big storm blowing, and 30 foot
  seas running.  But the big Boeing was successfully landed without
  damage.  The seas were so high, it was almost impossible to transfer
  the occupants to the Cutter.

I don't think anything is certified for takeoff in hurricane conditions, seaplane or not.

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text:
Why can’t the CF34 use wide-cut fuels?
The General Electric CF34 high-bypass turbofan (used primarily on the Bombardier CRJ and Embraer E-Jet regional jets), unlike the vast majority of jet engines (both civil and military), is not certified to burn so-called “wide-cut” fuels (gasoline-kerosene blends, nowadays used mainly in cold-weather operations), such as Jet B1 and (formerly) JP-3 - only “narrow-cut” fuels (straight kerosenes), such as Jet A-1,2 Jet A, JP-5, and (formerly) JP-1.  In contrast, CF34s in military service (known to the military as the TF34, and used mainly on the A-10 ground-attack aircraft) do just fine with wide-cut fuels (an absolute necessity back when aircraft so engined entered military service, as the go-to military jet fuel back then was JP-4, aka Jet B with the military’s stamp of approval).
This is a bit surprising, as:

it renders CF34-equipped aircraft unable to operate in very cold places (wide-cut fuels, having much lower freezing points and viscosities than the straight kerosenes, remain useable down to much lower temperatures than their narrow-cut ilk), and
there doesn’t seem to be any obvious technical reason for such a restriction, seeing how, as mentioned above, military CF34s are perfectly happy to run on wide-cut fuel.

Is there some critical difference between the military and civilian versions of the CF34 that renders wide-cut fuels compatible with the former but not the latter (maybe a gasket somewhere in the civilian CF34 that’s dissolved by the gasoline in wide-cut fuels?), or did General Electric simply not bother to certify the civilian version for use with wide-cut fuels?

1: Formerly also often known as “JP-4”.
2: Also often known as “JP-8”.

What is your source for saying it's not certified for wide-cut fuels?

Actually, the CF34-3B1 engines are certified to use wide-cut fuels, JetB/JP4. This comes straight out of the AOM for the CRJ200. I would venture to guess that all variants from CF34-1A to 3B would have the same certifications.
The CF34-8C5 engines on the other hand are not, again straight from the AOM for the CRJ 705/900. CF34-8C1 to 10E also likely have similar certifications for fuel certifications.
The difference is likely more to do with cost to certification. Seeing as wide-cut fuels aren’t overly common, except in the most remote locations north of 60 degrees of latitude, GE probably said why bother spending the extra money. JetA1 is good to -47*C/-53*F so for most commercial aviation, it’s more than sufficient. Every aircraft I’ve flown to date had an on-ground operating limitation of -40*C anyway.

Wide-cut fuel is more flammable, so in a crash Jet-A would be safer.
CRJs can run on both just fine but from the limitations section of the manual, wide-cut is limited to ferry flights (non-revenue), so no people besides the crew.


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Why are the B-47’s inboard and outboard engines mounted so differently?
The B-47 has six underwing GE TG-190 engines, two on each inboard pylon and one on each outboard pylon.
The inboard engines are mounted on a long pylon that extends well below and forward of the wing, such that the inboard-engine tailpipes are (longitudinally) just about even with the wing’s leading edge.
The outboard engines, in contrast, are mounted on much shorter pylons and fit snugly against the underside of the wing, with their tailpipes extending aft of the wing’s trailing edge; they’re placed so far back that their pylons even protrude beyond the trailing edge.

(Image originally by the United States Department of Defense, via Balcer at Wikimedia Commons; modified by me.)
Why the long pylons and forward mounting for the inboard engines, versus the short pylons and rearward mounting for the outboards?

Image source
From Wikipedia:

The wing's flexibility was a concern, as it could flex as much as 17.5 ft (5.3 m) at the tip, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down.

And

...with the twin inboard turbojet engines mounted in neat pods, and the outboard engines tacked under the wings short of the wingtips.... This arrangement reduces the bending moment at the wing roots, saving structural weight. The mass of the engines also acts as counter-flutter weights.

The B-47 required six engines of the type with maximum thrust that was available at the time. Four were mounted in twin pods, at about 1/3 span in order to provide decent bending and flutter relief to the thin wing. The remaining one was mounted past the fowler flaps almost at the tip - a cluster of three engines may provide cooling issues for the middle one.
In the photo it can be seen that the thin wing bends down considerably when on ground, and that there is no room for a forward extended pod at this place.

I'm not seeing a bendy wing in that photo; I'm seeing a wing with considerable anhedral.

Yes, on the ground. In air the wing tip flexed up considerably, up to 5.3 m.

The forward pylon mounting of most jets with wing mounted engines is to use the engines' mass to move the overall center of mass of the wing forward, closer to or ahead of the torsional axis of the wing box.  Boeing discovered this helps with aerolasticity related issues with long, flexible, swept wings.  A bit like mass balancing a control surface, but on the root structure.
The outermost engines, being so close to the tip where the chord is short, have to be farther aft or they would move the wing's center of mass too far forward in that area.

Quite so John. The B47 incorporated many new (to the US at least) design & construction elements. It is credited with establishing podded jet engines on pilons for swept wings, although the Germans had first investigated this years earlier. Swept, flexible wings on such a large aircraft was fairly unprecedented, AFAIK. The B47 had very flexible wings, & the outer engines were mounted at a relatively weaker section, particularly with regard to twisting. (early drawings had them mounted on the wing tips)

And the outboard engines had to be mounted on shorter pylons as the wings drooped a lot on the ground and longer pylons would have risked them dragging on the tarmac.

Interesting that the combined thrust of the B-47 podded twin was about half of the 737 Max LEAP fan jet. The 2 on the ends added 14,000 lbs more thrust.

"... to move the overall center of mass of the wing forward" - @john-k: Does this explain the similar use of long pylons on the inboard engines, and very small pylons on the outboard engines, of the B-58 Hustler? Image


text:
Why can’t the F28 be deiced with one or both engines running?
Recently, I was contemplating Air Ontario Flight 1363, which, despite heavy, wet snowfall while it was on the ground at CYHD, could not be deiced before takeoff, as it arrived bearing an inoperative APU, necessitating (as CYHD, at the time, had no ground-starting equipment available) that one of its engines be kept running the whole time the aircraft was on the ground, and hot-deicing an F28 (deicing it while one or more engines are running) is forbidden.  The aircraft eventually took off, and, predictably, crashed almost immediately due to snow and ice accumulation on its wings.
Wikipedia claims that the prohibition against hot-deicing an F28 is to prevent vapours from ingested deicing fluid from entering the cabin ventilation system via the engine bleed air used to heat and pressurise the cabin, but the engine bleeds can easily be turned off before deicing the aircraft, and any residual deicing fluid left in the engine after deicing can be dealt with by letting the engine run for a few minutes to flush out its passages before turning the bleeds back on (and, if necessary, one could start the engine on the side not being deiced at the moment, and route bleed air from there into the other engine to blow out any deicing fluid, or vapours therefrom, that did somehow make it into the bleed-air passages).
What gives?

You'd have to find an F28 driver or mechanic that knows the Spey engine's idiosyncrasies.  Best guess is the bleed system configuration could collect fluid between the compressor bleed port and bleed shutoff valve and which couldn't be purged.

@JohnK: Even if that were the case, I still don't see why fluid pooled there couldn't be blown back out by routing bleed air from the other engine down those tubes (after starting said other engine first, if necessary).

I don't know how you could do that system wise (cross bleed starting just routes air from the live engine to the starter of the dead one, not to it's bleed port), but in any case, bleed used for air turbine starting normally comes from a higher pressure port farther down the compressor than where it's normally tapped for A/C.

The real question should be: why were the pilots flying the F28 into potential ground icing conditions with an inoperative APU, and therefore no ability to de-ice on the ground? Most likely, pressure from management to meet schedules. In blessed hindsight, they should have aborted the flight.

@JohnK: Pressurise the bleed-air manifold using the live engine, then, without starting the shutdown engine, open the bleed valves for the shutdown engine (all the while keeping the valves to the air-conditioning packs shut), maybe?

The Fokker F28 can be de-iced with engines running, but this was not permitted by the Air Ontario manuals.
The Wikipedia article does not list a source for their claim:

the Fokker F28 aircraft is never supposed to be de-iced while the engines are running because of a risk of toxic fumes entering the cabin of the aircraft

One can however find all details in the final report, which shows an excerpt from a memorandum distributed by Air Ontario's director of maintenance:

This memorandum was not known to all Air Ontario pilots:

The body of evidence on this point does not support a conclusive
  finding, one way or the other, as to whether Captain Morwood and/or
  First Officer Mills received the Fokker cold-weather operation publication
  or the Mauracher memorandum, which contained, among other
  important information, the crucial proviso, "NEVER: Spray while main
  aircraft engine's are running!!!" (Exhibit 317, p.3). What is clear is that
  a specific pilot bulletin was never disseminated on this point, and there
  is strong doubt, based on the above-mentioned evidence of Air Ontario
  pilots, as to whether pilots Morwood and Mills had received the Fokker
  cold-weather operation publication.
Air Ontario pilots Hansen, Wilcox, Stoger, and Monty Allan testified
  that they were not aware of specific restrictions against spraying the F-28
  for the purpose of de-icing while one engine was running. Only Captain
  Maybury, who had received the Fokker publications, was aware of such
  restrictions. In the absence of specific instruction or a company policy on
  this point, Air Ontario pilots who were questioned in this regard (in
  particular, pilots Hansen, Wilcox, and Allan) testified that they considered
  the practice of de-icing with a main engine running unsafe,
  because of the risk of ingesting glycol into the engine and the danger of
  having a person in a de-icing bucket in close proximity to a running
  engine. However, the evidence of experts in the fields of aircraft ground
  de-icing, aircraft engines, and cold-weather operations indicated that
  aircraft ground de-icing is routinely performed in Europe and the United
  States with engines running. Clearly, an operational matter of such
  importance requires a standard company policy that is made explicitly
  known to all pilots and operational personnel.

(final report, p. 658f., emphasis mine)
This implies that de-icing with engines running is possible, but without
proper guidance or procedures most Air Ontario pilots would advise against
it. The report summarizes its finding on this matter:

The report does not say however, why the original Fokker publication prohibits
de-icing with engines running.

AAA!  THE GREENGROCER'S APOSTROPHE!!!

1,825 pages... that final report is longer than War and Peace! :-p


text:
Why didn’t Boeing ramp 737NG production back up in response to the 737 MAX groundings?
The predecessor of the currently-grounded 737 MAX is the 737 Next Generation (737NG), produced from 1997 through 2019.  The NG family (comprising the 737-600, -700, -800, and -900) is extremely similar to the MAX, with the main differences between the two families being the engines and mountings thereof (the 737NG uses the GE/SNECMA / GE/SAFRAN / CFM International CFM56 engine, housed in distinctive hamster-pouch nacelles, while the 737 MAX uses the Franco-American consortium’s newer, larger LEAP engine and mounts it in bog-standard round nacelles which are located further forward and higher above the ground than those of the NG1), the landing gear (the NG uses the same short-legged landing gear dating all the way back to the 737-100 in 1967,2 while the MAX’s landing gear is longer than that of all prior 737s3), and parts of the flight-control system (the MAX’s larger, more-forwardly engines cause it to handle slightly differently than the NG at very high angles of attack, so its pitch trim system was modified to introduce additional nose-down trim at high attack angles in order to make the MAX handle more like the NG and thereby ease cross-training of pilots; unfortunately, the modified trim system proved susceptible to inducing an uncontrollable nose-down trim runaway in the presence of certain sensor failures, which is why the MAX is currently grounded).
When the MAX was grounded in mid-March 2019, the NG was still in large-scale production, although the assembly lines were ramping down in preparation for a switch to producing the MAX and only the MAX; it would not have been particularly difficult to ramp production back up to full scale, enabling Boeing to continue producing, selling,4 and delivering airworthy 737s for the duration of the MAX’s grounding.  Boeing, however, did not do this, and instead continued to taper off NG production (finally ending it entirely at the end of 2019), while continuing to build and accumulate undeliverable MAXes until being forced to put production on hold in January 2020 for want of space to put them (thus marking the first time in the half-century-plus history of the 737 that production has ever stopped completely).
Why didn’t Boeing reincrease 737NG production in response to the MAX’s woes, rather than shutting it down completely and leaving the company with no deliverable 737s?

1: Which is part of the reason why the MAX, despite its larger engines, does not require hamster-pouch nacelles.
2: Which is the reason why the NG does require hamster-pouch nacelles (as did the 737 Classic before it), as the ground clearance beneath the wings (designed for the 737 Original and its slim Pratt & Whitney JT8Ds) would be too small to safely mount the CFM56 otherwise.  Even with the hamster-pouch nacelles (the pouches of which contain the engine accessories, moved from the bottom of the engine to the sides), SNECMA still had to shrink the diameter of the engine’s fan in order to get an acceptable amount of ground clearance, which is why the CFM56s used on the 737 Classic and NG are slightly smaller and less efficient (due to the decrease in bypass ratio resulting from the shrunken fan) than those used on the KC-135R/T, DC-8-70, and A319/20/21ceo.
3: Which is the other part of the reason why the MAX, despite its larger engines, does not require hamster-pouch nacelles.
4: Possibly, although not necessarily, involving offering MAX customers the opportunity to switch their orders to NGs, potentially along with other incentives to sweeten the deal (such as refunding them the difference between the prices of an NG and a MAX, or slightly more [thus turning an undeliverable MAX order into a discounted, but deliverable, NG order], and/or giving them a discount on future purchases of Boeing aircraft).

I think Boeing will do whatever the airlines want, and the  airlines clearly didn’t hint at this idea.

MAX was (is) clearly superior to NG, why would operators settle for less? It is my understanding that Airbus 320 NEO is superior to NG, thats why MAX was rushed into production in the first place - to beat the NEO. Boeing also clearly had no idea of how long the geounding of the MAX would take, the first comments were, IIRC "we'll have it fixed in a few weeks"...

...and that seemed by no means unreasonable, for something that was apparently mostly just a stupid bug in a very simple piece of software.

(Which is not to say that the MCAS really makes sense. The real problem is that the whole idea of the 737MAX design is broken.)

@leftaroundabout the design is fine. The problem is using one sensor.

"Good afternoon, Sean, and welcome back to Mercedes Benz of Everett! We're so happy that you've ordered your S55 AMG from us in black-on-black, let me walk you to the delivery area where we have your silver-on-champaign S430 prepped and waiting for you to drive home today!" Yeah, that wouldn't go over well, would it?

@Firefighter1 I kind of agree with leftaroundabout on this. Modifying how the plane behaves in a way that needs to be covered by software and telling everyone this means nothing has changed so you can sell it without extra delays is a bad idea. You missed that he did not say the design is broken, just the idea behind it. Which it is.

@FreeMan: ...I don't follow.

If you go to a car dealership to order a specific car (a 737MAX with this seating configuration, these avionics, etc.), that's what you expect to get. If you arrive to pick up your car and they show to something else that's "equivalent", but not what you ordered, you're not going to be particularly happy about it. Boeing may face difficulties trying to get airlines to accept all their newly built 737NGs when the customers ordered MAXs. i.e. ramping up NG production wouldn't fill the customer's desire for a MAX.

This comment put it more succinctly...

@FreeMan: If someone's selling you something, and unforeseen circumstances render the specific item ordered unavailable, isn't it pretty much standard practice to contact the buyer, tell them that what they've ordered isn't available for some reason, and ask them if they'd like to switch their order to the closest thing the seller's got available (usually at some degree of discount)?  'Cause that would be a closer analogy than someone walking into a dealership at pickup time and only then finding out that what they wanted wasn't there.

There are several reasons why.
First, it takes an awfully long time to make that kind of switch. While one can switch an assembly line for a product, that works rather differently for (say) transistor radios and Boeing 737s. The assembly line for an airliner winds back very, very far, and nothing happens or can happen immediately, or even very fast.
That would have made it very difficult, even if Boeing wanted to do that.
And here's a second reason: they didn't want to. They had orders for the MAX. If they were late in delivering those aircraft, they could be subject to financial penalties. Boeing agreed to deliver the aircraft, so had to build them. At the same time, they weren't facing overwhelming demand for other 737s that could possibly have made it worthwhile to do it in the circumstances.
A third reason: Boeing had every expectation that the aircraft would soon be allowed to fly again, and continued to operate under that expectation (for much longer than many observers shared the same expectations, it has to be said).
A fourth reason: it would have been bad for business to do that. Doing what you suggest would have sent out the most appalling signals, i.e. that the company had lost faith in one of its most important products. In a way, they simply had to hope that things would turn their way sooner rather than later, and the option of turning away from that would have had tremendous costs. (You might argue that it was a gamble that didn't pay off; I think that would be fair enough to say.)
And finally, I think that Boeing's slow responses at many points in this story look like the behaviour of a company in shock. On multiple occasions, the company has found itself struggling to keep up with and make sense of events, even if they been events of their own making. Quite possibly the company's decision not to do as you suggest, or at least not to have done it much sooner, will turn out to have been a very bad one, but unfortunately, it will also have been only one in a long chain of them.
By the time I make several bad decisions in a row in a crisis, it's pretty much guaranteed that the next one I make will make everything even worse. Organisations also fall victim to that pattern.

Remember Nokia? They used to rule the cellphone market with about 40% share. Then, in just a few years with a series of bad strategic descicions: all gone.

Airlines need to diversify, apparently. They need similar assembly lines as the auto industry to accommodate multiple models simultaneously.

In this case divesity=cost. The "laws" of airliner markets and production are very, very different from the autoindustry. Sure, they could copy eachother, but in both cases, this would end up in products so expensive, they would not be bought.

@Skyhawg GM sells ten million vehicles per year. Boeing comes in at under a thousand.

@Skyhawg, there are many good reasons why all the low-cost airlines fly only one type each. Aircraft are not like trucks where a driver used to Mercedes truck can just climb in Scania one and drive off. Pilots need special training for each type. In fact it was the desire to make the new model behave the same as the previous, so the pilots wouldn't need conversion training, when aerodynamics disagreed, that started this whole issue. And that's before you even consider maintenance and stocking parts. Even spedition companies tend to stick to one type for easier maintenance.

One factor, probably the biggest factor in my opinion, is that a production program is a ship that takes a year or so to change direction and the decisions and planning for the ramp-down were taken long before the event.
Think about the supply chain cascade that has to happen to increase the number of a particular gee-gaw like an actuator of some kind; LRU supplier has to ramp up, which means its sub suppliers have to ramp up, which means their sub suppliers have to ramp up, which means... well I should be down to nuts and screws at this point.  Add a couple of months up each step in the chain and pretty soon you're looking at 6 months to a year before anything happens.
On top of that, this was probably unpopular with the airlines themselves, and the crisis itself has a nebulous end date (should all be resolved in 6 months... ooops pushed back to 9 months.... ooops pushed back to a year... and so on).  No one said, "this is going to take 2-3 years and we need to plan for that now".

"this was probably unpopular with the airlines themselves" - more to the point, if the airlines had wanted more NGs they would have ordered them, and there wouldn't have been a MAX program to screw up at all.

The short answer is money. The market dictates what Boeing builds. The demand has to come before the supply on something that expensive. The NG production line had orders to fill. After the orders were done, there was no need for the NG line. If serious demand for the NG returns, it might make financial sense for the NG production line to return.


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Why does the Comet's horizontal stabilizer have so much dihedral?
The Comet's horizontal tail has a noticeably extreme amount of dihedral compared to that of, say, a 707:

(Image by MilborneOne at Wikimedia Commons.)
Note how the Comet's tail has about twice as much dihedral as that of the 707 in the background, especially considering the Comet's quite low wing dihedral.
Why such a large amount of dihedral on the Comet's tail?  Does this have anything to do with the Comet's tail being unswept (whereas most later jetliners have horizontal stabilizers that have almost as much sweep as the wings, if not more)?

The Comet's engines are in the wing, higher than the 707's pod-mounted engines so I'd guess the dihedral is to keep the elevators out of the engine wake

As mentioned in this question, the two major factors are:

Lifting the tail plane out of the jet efflux area. Placement of the stabiliser root is structurally optimal when at fuselage mid, dihedral can then lift the surface out of the efflux stream. Note that the Comet has a relatively straight aft fuselage cone, which is better for low drag. And like @DaveGremlin mentions in his comment: the jet engines on the Comet are higher than those of the B707 and need to be lifted up higher.
Reducing sudden main wing wake effects: with a pronounced dihedral, there is always some of the tailplane sticking out into a less affected streamline.

The tailplane dihedral does contribute to roll stability, like main wing dihedral does as mentioned in this answer.


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Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing?
The DC-8 is one of only a few jetliners that can safely use reverse thrust in flight; even so, reversing the inboard engines (the outboard engines can only be reversed on the ground) while flying still produces extensive airflow disruption over the wing’s upper surface in the vicinity of the reversed engine(s), as detailed in this excellent PDF documenting a series of flight tests carried out by NASA on their DC-8-70.  A significant area of separated flow extends from leading to trailing edge (wider at the trailing edge) whenever the inboard engines are reversed below approximately mach 0.88, and becomes very large (covering over a third of the upper wing surface) below approximately mach 0.6; additionally, at mach numbers of 0.6 and below, an area of full-on reverse airflow appears over the trailing edge, in line with the inboard engine.
This would make perfect sense if the engines extended aft of the trailing edge, in which case the reversers would blow air forward and over the wing, but that isn’t the case for the DC-8; instead, the DC-8’s tailpipes (even on the DC-8-50 and -60, with their long, cigar-shaped JT3Ds) are placed under, or just behind, the wing’s leading edge.  If anything, one would expect the use of reverse thrust on a DC-8 to strengthen the airflow over the wing, instead of disrupting it, due to all that forward-blowing air piling up around the engine and under the leading edge and helping to partially block air from passing under the wing, forcing it to go over the top instead.  So why does in-flight thrust reversal on a DC-8 instead produce large areas of airflow separation (and, at lower speeds, even airflow reversal)?

That's not surprising at all.  What the pictures of the tufts show, and exactly what I would expect to see, is a massive injection of turbulent reverser air into the upwash just ahead of the LE, disrupting everything behind it, and which would include a significant component of flow over the top of the wing.  The tufts show some straight and some pointing forward, which suggests a layer of very turbulent air with random eddies.  Like this:


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How do aircraft making HF transmissions avoid stepping on other HF users?
HF radio (High Frequency; the portion of the radio spectrum extending from 3 to 30 megahertz (MHz),1 although only some of this slice is available for aeronautical use) is the backbone of long-range aircraft communications, as HF transmissions are reflected off the Earth’s ionosphere, allowing aircraft to communicate with ATC stations well below the horizon.  In contrast, VHF (Very High Frequency; 30 to 300 MHz, although, again, most of this is unuseable for aircraft transmissions) and UHF (Ultra High Frequency; 300 MHz to 3 gigahertz [GHz], although, once again, only a small fraction is available for aircraft to use), the backbones of short-range communications with and between civilian (VHF) and military (UHF) aircraft, pass straight through the ionosphere and out into space, making them line-of-sight-only2 (and, thus, useless for contacting aircraft located, say, out over the middle of the ocean).4
In the context of aircraft communications, another difference between HF (on the one hand) and VHF and UHF (on the other) rears its head.  Unlike VHF and UHF, where the space allocated for aircraft communications is in one neat, contiguous bunch, commonly known as the airband (117.975-137 MHz for VHF, 225-399 MHz for UHF5), the aircraft-communications portion of the HF range is split up into lots of separate bands, scattered throughout the majority of the HF spectrum; I count thirteen non-contiguous chunks of HF spectrum allocated to “aeronautical mobile” use, at least according to this PDF.6  With VHF and UHF, one can simply build a radio whose tuning range is restricted to the VHF or UHF airband (much like how consumer radio receivers have tuning ranges restricted to the AM or FM broadcast bands7), but an aeronautical HF radio is, by necessity, going to have a tuning range that covers a huge amount of spectrum allocated to non-aeronautical use (unless you want to only be able to communicate on frequencies lying in one specific HF band out of the thirteen available, which is, for obvious reasons, a poor solution), posing a great risk, if accidentally tuned to the wrong frequency, of causing interference with non-aeronautical HF users.8
How do aircraft and air traffic controllers communicating on the HF bands avoid accidentally stepping on other HF users in this manner?

1: Although, in aeronautical use, the HF range extends down to 2.85 MHz, into what is technically the uppermost portion of the MF (Medium Frequency; 300 kilohertz [KHz] to 3 MHz) range.
2: Technically, VHF and UHF transmissions can be picked up somewhat below the horizon (out to approximately one-third-again line-of-sight range), as radio waves in these frequency bands are refracted by the earth’s atmosphere to some degree, but the point remains that these waves don’t reflect off the ionosphere.3
3: Okay, sometimes they do, but only under extremely unusual ionospheric conditions.
4: This isn’t as rigidly true nowadays as it used to be, as the advent of satellite communications has started to allow aircraft to communicate over very long distances with each other and with ATC at previously-ordinarily-nominally-line-of-sight frequencies, but HF radio still remains quite important.
5: ...which results in the lower three-sevenths of the UHF airband actually being in the upper VHF range!
6: I am aware that this PDF is a few years old; however, given the degree of extreme conservatism where anything that could possibly affect air travel is concerned, the bands of spectrum allocated to aircraft use probably haven’t changed much since then.
7: MF and VHF, respectively, in case you were wondering.
8: The purposes to which the HF spectrum between the various “aeronautical mobile” bands is allocated include radar, amateur radio (including a few marked “amateur satellite” - apparently not all of the HF spectrum bounces off the ionosphere rather than passing through), ship-to-shore and ship-to-ship communications, several frequency and time signals, the numerous shortwave broadcast bands (nine lying in the portion of the HF spectrum that concerns us here, plus one more located beyond the upper end of the highest-frequency aeronautical HF band), and even one radioastronomy band, in addition to the dozens of bands whose allocations are labelled in the aforementioned PDF as simply “fixed” or “mobile”, without further elaboration (except for the fact that a number of the latter specifically exclude “aeronautical mobile” usage).

We already have a really nice and concise answer to the actual posted question, but the long exposition in the question text shows a few points about use of MF/HF radio in general which are a bit unclear, so let me begin with this wall of text:
Term HF itself:
Although we have the traditional separation of radio frequencies into wavelength-based bands, that distinction isn't actually very useful in real life. This resulted in unofficial extensions to some bands.
Therefore, the "upper" part of the medium frequency band, with upper meaning above the medium frequency broadcast band, got absorbed by the high frequency band. It's quite common for commercial HF radios to start transmitting somewhere between 1.5 MHz and 2 MHz and continue up to above 25 MHz. In German language, for example, there's a term Grenzwelle (meaning border-waves), which represents the region between 1.6 MHz and 4 MHz, since it tends to experience similar propagation characteristics.
Discontinuous HF air bands:
There seems to be a bit of surprise about the discontinuities in the HF air bands, but that's not a bug, it's a feature.
The actual HF frequency planning is very complicated. The propagation on the HF depends on the time of the day, on the time of the year, on the location, which all which affects the amount of exposure to the Sun a region receives. And then, there's the local space weather which affects the HF propagation, and the long-term solar cycles which affect the space weather, and therefore affect the propagation of radio waves on the high frequencies. It's easy to say that the HF radio reflects of the ionosphere, but it's difficult to say at which altitude, and to get the actual frequency assignments from that.
So to stop writing very generally and to get a bit more specific:
Lower frequencies work good at night, higher work better during the day.
Lower frequencies work better during the winter, higher work better during the summer.
Lower frequencies often allow shorter ranges, which higher frequencies allow higher ranges.
The handwaving in the above statements is intentional. Some interesting tools to take a look are for example the Hourly Area Prediction maps based on ionosonde measurements. Australian Space Weather Services have them available here.
The charts mentioned above show which frequencies are suitable for communication depending on the distance. Let's take a look at a chart made at the time I'm writing this answer:

Let's say I'm in Belgrade, and I want to talk to someone in Portugal. I'd need a frequency assignment in at around 12 MHz to bridge the distance. On the other hand, if I wanted to talk to France, I'd be looking at 8 MHz to 10 MHz, and if I needed to communicate locally, I'd be looking at somewhere around 4 MHz to 6 MHz.
Such planning is quite interesting in case you want to use a traditional commercial HF station, such as Stockholm Radio for example. They'll have a set of guard frequencies they listen to, usually one on every band they cover, and a set of operational frequencies they'll use to reduce load. Here's an interesting article (STORadio Pilot Refresh) from them about HF radio.
Contiguous VHF air band:
Let's take a look at sizes of dipole antennas (the point can be extrapolated for other, more common types as well)) in free space for the VHF air band. One of the many calculators says that at 118 MHz, we need a dipole antenna which is around 120 cm long. On the other hand, at 137 MHz it's around 105 cm long. That's a difference, but nothing that can't be solved. This makes it very attractive to have a contiguous band on the VHF.
On the other hand, on the HF, we can't have a contiguous band, due to propagation. If we take a look a the antenna sizes for the highest and lowest guard frequency for the abovementioned STORadio, we have 40.826 m for the 3494 kHz, on one end of the spectrum, and 6.146 m for 23120 kHz the other end of the spectrum. In practice, mobile HF antennas, be they on a ground vehicle, ship, or an aircraft, are often made with trade-offs in mind, trying to work okayish on one of the more important bands, and having a so-called antenna tuner to match the impedance mismatch, enabling the antenna to work on other frequencies. The reduction in antenna efficiency is usually off-set with large enough transmit power.
So to sum this section up:
We don't want to have contiguous HF frequencies, due to HF propagation, and due to different frequencies, we have to make trade-offs on the radio set-up. On VHF, we have technical reasons which make contiguous frequencies attractive.
Finally, I'll rant a bit about the culture on the HF:
First of all, the statement made in the question claiming that the radio has to either continuously cover all HF bands, or work on only one is incorrect.
 Let's take a look a the maritime mobile radio service. It also has VHF and MF/HF sections. The VHF radios are channelized, just like air band radios, but instead of showing the frequency, and channel step, they show the channel name. The HF radios behave the same way: There are the so-called ITU channels. For example, the channel 1201 is the first channel in the 12 MHz marine band, and has a set frequency for the ship to transmit, on which the coastal station listens, and a set frequency on which the ship listens and the coastal station transmits. Depending on the radio, you get a channel knob, which you use to change the channels, or you can use the menu to select the band, and a channel within the band.
For amateur radio, it's similar: There's a band-selection menu, or a set of buttons, which allow you to select a band, and then you can chose the frequencies within it to transmit.
On land-mobile HF radios, the situation is a bit different, since there's usually a set of pre-programmed frequencies to which the radio can tune.  Quite often, they'll even have methods to automatically chose the best band from the list of available frequencies.
However, what's common to all 3 is that, usually there's a more or less difficult way to "unlock" the radio. The radios usually cover almost entire HF range, and are limited "only" by the software to the licensed frequencies.
Finally, I'd like to address the great risk statement, which is also, in my opinion a part of the culture.
Namely, it's important to understand, that when using the ionospheric refraction to communicate over HF (as it's intended to be used), HF radio is not reliable! There's an effect called fading, which leads to differences in signal strength. It can vary with time, and with frequency. Sometimes, it's very slow, compared to the rate of change of the signal, and sometimes, it can be very fast. Here's an example of fading on an AM broadcast station in the 31 m HF band:

The top part of the picture, right below the 9500 (frequency in kHz), shows the current spectrum, with each grid being 2 kHz wide and 10 dB of signal strength tall. The bottom part with the red line shows the so-called waterfall, view as if you were looking at the spectrum from above, showing around last 5 or so seconds. Yellow is the audio signal of the station, with blue stripes representing the fading, which was around 20 dB in intensity, a reduction of signal power of around 100 times, on those frequencies, which is enough to reduce a good quality audio to unreadable level.
There's interference from other HF users, which can be intentional and unintentional.
Here's an example of a typical day (well, night actually) on an HF broadcast band:

We can see several broadcast stations, running AM, we can see a regular radar radiating right next to a couple of very weak AM stations, and we can see the usual behavior of ionosondes, covering very wide spectrum very quickly, with very little regard to other users. It's actually sweeping continuously through frequencies, the snapshot dots making a line are artifacts from the radio.
Then, there are other intruders from out of band: It's not uncommon to see stations operating outside of their allocated frequency bands intentionally. It's not uncommon to see encrypted military HF communications in amateur radio band and radars in amateur bands, and it's also not uncommon to see pirate broadcasters in marine bands as well, transmitting either music, or chatting, or transmitting propaganda. The situation is so bad, that the ITU even has "Regular Monitoring Program in frequency bands between 2 850 kHz and 28 000 kHz" just for that. You can even take a look at their reports. It's quite common to see descriptions such as GERMAN SW PIRATE, or TWO MALES IN USB causing interference to digital selective calling frequencies, or INTRUDERS LSB and so on.
So to sum up the risk part: On HF, you'll never know if you're going to be struck by a radar, by a numbers station, by a pirate, or by regular fading. Therefore, you need to take steps to recover from such situations and to be able to continue on. This leads me to conclude, that the risk isn't great at all. If you actually transmit for a bit outside of your own assigned band, nothing is going to happen. Yes, you might jam someone for a bit, but fading could have made the same effect as well. The jammed party is just going to have to say the last relation again and that's more or less that.
By the way, there is a trend of replacing HF communications with satellite communications, which seem to suffer much less problems compared to HF.

Interesting.  I don't recall running into issues like this, but then I was doing this 30 plus years ago now.  Given that, the original question/concern really shouldn't be about aviation users stepping on others, (when operating on an assigned ATC frequency) but getting stepped on by unauthorized users...

If those radio stations are actually in the HF region, wouldn't they have to be shortwave stations, not AM?  The AM broadcast band is in the middle of the MF area.

@Sean: The term "AM" as used in this answer refers to a modulation mode, not a frequency band. You can use AM modulation in any frequency band you like.

@GregHewgill: Ah, OK.

Oceanic airspace is divided into MWARA, Major World Air Route Areas. Each of these areas defines a set of HF frequencies that are used for air-ground communications. Within each area, the frequency used by aircraft at any given time is usually predetermined by the time of day based on propagation characteristics of the given frequencies (and published in a schedule).
For example, this page lists hours of operation for some areas: North Atlantic (NAT) MWARA Profile and Frequencies.
Typically a ground station will monitor all of their MWARA frequencies simultaneously. (This is at least true for the South Pacific area, which I'm familiar with.) So if an aircraft tries to reach a ground station and cannot, then they might try a different frequency, or just try later. Frequencies other than the assigned MWARA ones are not monitored.
The ground stations use equipment that cannot tune to a frequency other than the assigned ones (e.g. five in the case of the SP area). I'm less familiar with the HF equipment installed in the aircraft, but since there is a fixed list of published frequencies worldwide (a few dozen), the frequency may be selectable from a predefined list.

Good answer.  The short answer is frequency deconfliction.  I used to be an overwater airborne HF radio operator and we had a booklet of frequencies published for each of the oceanic control sectors we crossed.  We would simply dial in one appropriate to the station we were calling.  No worries about conflicting with other non-aviation users because they ought not to be on that channel.

What do they do if the single aviation HF guard frequency (2.85 MHz) isn't propagating and someone has an emergency while beyond VHF range? (Ignoring satellite communications here.)

@Sean: I'm not familiar with 2.85 MHz. I know that is not one of the frequencies monitored by HF aviation ground stations in the South Pacific, but I can't say what it might be used for elsewhere in the world.


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How do pilots avoid clouds under night VFR?
When flying under visual flight rules (VFR), pilots have to be able to see far enough away to be able to avoid running into other aircraft or the ground.  Therefore, VFR flight requires good visibility with a high (or no) cloud ceiling; this is known as visual meteorological conditions (VMC).  It also requires that pilots avoid clouds like the plague, since clouds (on Earth, at least) tend to be on the opaque side, and a cloud that you can't see through could potentially be hiding something that it would be very bad to run into (such as someone else's aircraft).
Avoiding clouds is easy enough during the daytime, if the weather is good (the big white things tend to be fairly visible as long as you aren't actually inside one), but, in many countries (including such biggies as the US, France, and Australia), VFR flight is also permitted at night.  As clouds are not usually polite enough to illuminate themselves for the benefit of nearby pilots, they tend to blend in rather well with the dark sky (except sometimes when bright moonlight is present), the only hint of their presence being that they occlude background stars (which provides absolutely no information about whether those stars are being hidden by a small cloud close by, or by a big cloud further away, and depends on the stars being easily visible to begin with [which is not always the case, even at night]).
So how do pilots avoid clouds when flying VFR at night?

Moonlight is more common, and more useful for seeing clouds, than you might think.  Other than that, you use eyes, weather forecasts, and common sense to avoid flying when there might be clouds you can't see.

Clouds have a nasty habit of popping up from place to place even in good weather - no amount of weather-forecast watching can assure the absence of clouds.  And, while moonlight might well be common, the lack of moonlight is also common - what if it's the new moon or a thin crescent, or the moon is obscured by a high-altitude cloud ceiling, or the moon hasn't risen yet / has already set for the night?

Depends on where you do your flying.  In most of the western US (where I've done the great majority of my flying), clouds are fairly predictable - e.g. if there are clouds on a summer night, they are going to be cu-nimbs full of lightning.  Then, as with those other situations, you stay on the ground :-)

I will caveat my answer by saying that most of my night flying has been under IFR, but because of that I have lots of experience going in and out of clouds at night.
It starts with a good weather brief, and don’t forget to ask for PIREPS that might cover areas further away from the field.  True enough the bases can be ragged, and little puffies can lurk in unexpected places, but if you know where the layers are called you are well on your way to avoiding being at the same altitude.
To add to one of the comments, moonlight is helpful, but so is ground lighting.  At night distant clouds can be difficult to see, but you can use ground lighting to gauge where the coverage is.  For example, if your eyes can trace a lighted road off in the distance, but at approximately 20 degrees below the horizon the car lights become blurry, then disappear, it is probably because of a cloud between you!  Angle away, or descend below the deck until the horizontal visibility range increases.
If you start skimming clouds you will know it immediately because your anti-collision light will blossom and reflect red all over the inside of the cockpit.  If you still have ground lights in sight and you know the clouds are stratified, I would start a gentle descent.  If there are a lot of vertical buildups and you pop into one and lose sight of the ground, make a level 180 and get out of there.  Most non-instrument rated pilots are taught to deal with inadvertent IMC in this way.
As I mentioned in my answer to your other question, make sound risk decisions.  If you are uncomfortable with the weather conditions or your ability to remain clear of clouds, then choose to not fly VFR at night.  But once you get a few night flights under your belt I think you will find that they can be very enjoyable when conditions are good.  It opens up a whole new perspective on things!

One thing you can do to minimize the risk of cloud encounters at night, besides the obvious one of staying below reported or forecast cloud layer altitudes, is to maintain a positive dew point spread in the air mass you are flying in, to the extent you can, since cloud usually won't form until the spread becomes zero.
So if the dew point is 8C and it's 20C on the surface, a 12 degree spread, I would avoid climbing any higher than an altitude that gives a 2 deg C spread, about 5000 ft AGL, which should give a 1000 ft margin below any cloud that may form.  Of course, if you end up flying into more humid air with a higher dew point, you're back to square one, so you need to take into account factors like potential changes in humidity (flying from over dry land to over a large lake for example).
Beyond that, it's a case of being good at making 180 degree turns in IMC when the ground disappears.  In night VFR, if you encounter a cloud the priority is to return to the clear air you were in just before, so you simply make a level 180 degree turn on the gauges and wait to break out, then descend (you want to avoid climbing/descending turns in IMC when you only have a couple hours of time under the hood).

Starlight, moonlight and light pollution from the ground are often adequate to see where the clouds are almost as well as you can during the day. But not always - if there's a high overcast to block the stars and a new moon and you're out in the back country or over the ocean, it might as well be IMC. Pilots have to use their judgement and not fly VFR at night even in VMC if conditions are such that they can't be reasonably assured of maintaining VMC.


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Can a normal PASSENGER pilot a plane with no training?
This is an extreme question, but I am daring it because I want to.
Situation :

All pilots, officers, die.
just say no crews can pilot a plane
no former pilot in the passenger

Only option is for a normal passenger to pilot a plane. Is this possible?

Without any help, and no prior introduction, it is highly unlikely in a transport or biz jet.  Too much complexity, too much a priori knowledge needed, and too many flight configuration changes to land the plane.  Just an opinion.
There have been lots of attempts to take the uninitiated and have them pilot a simulator to a landing. The outcomes are not favorable, unless there is some previous experience or there is very very active coaching. Every now and then, someone writes a paper or magazine article on this. There have been no large scale studies where more than a handful of lay people have been "tested."
So bottom line, somewhat informed opinion, no it is not likely that the uninitiated would be able to fly to a selected destination and land a plane that is in the transport or bizjet class.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Why does taxiing produce large lateral loads on the 747's engine pylons?
While reading through a bunch of old AARs recently, I found (in the context of a 1993 747 in-flight engine separation) this interesting tidbit:

[...]  Boeing engineers report that the modifications will increase the pylon's vertical and longitudinal strength. However, the modification will provide a slight, if any, increase in the structure's lateral load-carrying strength. Additionally, it was provided by Boeing engineers that the greatest lateral loads on the pylons normally occur during taxiing.  [Page 40 of the PDF of the aforelinked AAR, numbered as page 33; emphasis added.]

I'm having some trouble seeing how taxiing would be capable of generating any significant lateral loads on a 747's engine pylons; assuming that one is not attempting to drift one's 747, nor taking it excessively close behind another jet running its engines up for takeoff, taxiing it at a reasonable taxi speed should not produce any large lateral accelerations that would make the engines want to swing on their pylons, nor any significant lateral airloads that would tend to push the engines sideways relative to the wing.
Intuitively, one would expect that the situations placing the highest lateral loads on the engine pylons of a 747 (or, for that matter, any jet with engines mounted on vertical pylons1) to be those involving one or both of the following:

Large, abrupt accelerations from side to side (such as during severe turbulence, as experienced by [for instance] the aforementioned 747 whose engine separated in midair).
Flight at large sideslip angles (in which case the direction of the airflow experienced by the engines and pylons would be out of line with the longitudinal axis of said engines and pylons, generating considerable, though steady, lateral airloads on the engine-pylon assemblies).

So why would normal taxiing produce such large lateral loads on the 747's engine pylons?

1: For a jet with horizontal engine pylons (such as a DC-9 or CRJ-1000), one would instead expect the highest shear loads2 placed on the pylons to be experienced during abrupt pitch changes, high-angle-of-attack flight, large vertical gusts, or (in the mostly-hypothetical case of a jet with wingtip-mounted engines) high-roll-rate manoeuvring.
2: The loads that try to snap the pylon in two, rather than move the engine longitudinally or stretch/compress it out of/into the pylon.

Taxi gets you the highest turn rate.  Wouldn't the largest lateral load come from the engines trying to swing outward as you turn?

@Tristan: Even more: The fans are huge gyros, and when the aircraft does a fast turn, this means high torque around the vertical axis, which translates to high lateral force on the pylon. (Does anyone have the momentum of inertia of a 747 engine?)

The slip-skid ball indicates the ratio of "sideways" apparent acceleration to "upwards" apparent acceleration, in the aircraft's own reference frame.  In normal constant-airspeed wings-level flight (i.e. not looping or pulling out of a dive), or while taxiing, the "upwards" apparent acceleration is 1 G, so the sideways deflection of the slip-skid ball is a direct indication of the total lateral force acting on the aircraft and the total lateral acceleration experienced by the aircraft.
Turning at a high rate while taxiing will displace the slip-skid ball far to one side.  During high-speed taxi, and the portion of the takeoff and landing where the wheels are on the ground, steering corrections will typically involve low turn rates but may still cause a large displacement of the slip-skid ball due to the high speed involved.  In either case, the large displacement of the skip-skid ball indicates that a strong lateral force is being transmitted to the aircraft, including to the engines through the engine pylons. The fundamental origin of this lateral force is the sideload generated by the tires.
But we can't assume that in actual flight, a similar displacement of the slip-skid ball indicates that a similar sideload is being exerted on the engine pylons.
In actual flight, during a sideslip, the tires are no longer involved.  Instead, the sideways airflow interacts with the various surfaces of the aircraft to generate the aerodynamic sideforce that displaces the slip-slid ball.
Therefore, in flight, the lateral inertial loading created by the mass of the engine whenever the slip-skid ball is displaced, is at least partly offset by the aerodynamic sideforce generated by the engine nacelle.
Consider an aircraft banked to the left, with the slip-skid ball displaced to the left, showing that the airflow is blowing from left to right across the aircraft.  (A "yaw string" would blow toward the right.)  Which is generating more aerodynamic sideforce per unit mass-- the engine nacelles (including the engines contained within), or the rest of the aircraft?
If the former, the nacelles will be forced toward the right relative to the rest of the aircraft.  If the latter, the nacelles will be forced toward the left relative to the rest of the aircraft.  If the nacelles are generating exactly the same aerodynamic sideforce per unit mass as the rest of the aircraft, then the nacelles will not be forced either to the left or the right relative to the rest of the aircraft, so the pylons will not experience any lateral force at all.
Regardless of which of these three scenarios actually apply, we can be sure that unless the total G-load is quite high, for a given displacement of the slip-slid ball the lateral load on the engine pylon is higher if the aircraft is taxiing than in actual flight.

But the indication of the slip-skid ball cannot be directly translated to lateral load. Lateral load occurs because a force is trying to keep mass in a circle. Centripetal force is a function of turning radius and velocity. At taxing, the velocity is low, so although the resultant apparent gravitational force has a rather large angle to vertical, its magnitude should still be small.

It appears to me that the deflection of the slip-skid ball is a direct indication of the ratio between the "downward" component of the apparent G-load and the "sideways" component of the apparent G-load, in the aircraft's reference frame.  When the "downward" component of the apparent G-load is one, then the deflection of the slip-skid ball gives a direct indication of the sideways component of the apparent G-load.  This sideways component always reflects some real force, be it the sideload generated by the tires, or aerodynamic sideforce generated by sideslip.  .

On the other hand it is worth noting that during a high-G turn, a given sideforce in pounds, generated by sideslip, would result in less deflection of the slip-skid ball than we'd see in 1-G flight.

(Edited): So in short, I don't agree with your suggestion (as I understand it) that during low-speed taxi we could get a large deflection of the slip-skid ball even if the tires were only generating a small sideload. I think a large deflection of the slip-skid ball must always indicate a large sideload, except for special cases like reduced-G flight (ballistic arc) and negative-G flight. When you give descriptions of "lateral load" and "centripetal force", I think these are just two different ways of describing the same thing, at least in the wings-level case, as applies while taxiing.

Answer now edited to consider effects of high G-loading.


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Why do simultaneous-ops parallel runways need to be so far apart?
In the answers to this question about Denver's widely-scattered pinwheel of runways and this question about Schiphol's runway out in the middle of nowhere, it's mentioned that parallel runways are required, by both European and U.S. regulations to be over a kilometre apart in order to be able to have simultaneous departures or simultaneous arrivals on both runways.  In contrast, if one runway is departing an aircraft while a parallel runway is landing an aircraft, the runways need only be a few hundred metres apart.
Why is there such a large runway-separation requirement for simultaneous arrivals or simultaneous departures from parallel runways?  It can't be for aircraft separation, since an aircraft is allowed to land on a runway quite close to a parallel runway that is simultaneously departing an aircraft, despite the possibility of this resulting in a head-on collision if one or both aircraft drift off track, so what, in fact, is the reason for this?

Aircraft on parallel runways both go in the same direction, so that a departing plane and arriving plane would both be flying in the same direction.  There would be no head-on collision.

The simple answer seems to be for wake turbulence. This FAA document, ORDER JO 7110.308C, seems to say it is for wake turbulence reasons in para. 9:

The geometry of the approach, as well as the lateral separation between the two approaches and prevailing local meteorological conditions, provide the wake turbulence avoidance necessary for reduced separation dependent approach operations.

Figure 3 shows the aircraft arriving in a staggered fashion, 1 nautical mile diagonal separation with runways 2500 ft apart (parallel runways separated by less than 2,500 feet, also referred to as, Closely Spaced Parallel Runways (CSPR)).  The text also describes how the lead aircraft follow a lower glide path than the following aircraft such that  the lead's wake turbulence isn't a factor for the following aircraft.

Denver is not listed in table of airports with CSPR runways, are they wider apart and not considered CSPR?  http://aeronav.faa.gov/d-tpp/1805/09077ad.pdf#nameddest=(DEN)

This paper has  lot of interesting stuff as well, like the costs for seperation of large airplanes and the clearance zones needed around them,  https://www.faa.gov/airports/resources/publications/reports/media/large_airplanes.pdf

And this paper desribes some of the history of the runway seperations in para 3.7.  https://www.faa.gov/about/office_org/headquarters_offices/ang/offices/tc/about/campus/faa_host/rdm/media/pdf/report-sepstandardsworkinggroup.pdf

That document appears to be specific to dependent approaches.  Independent approaches require even more separation.

It is usually to guarantee sufficient aircraft separation when visual separation is not available (bad weather). See also Why is one of two parallel runways sometimes closed in foggy weather?

It can't be for aircraft separation, since an aircraft is allowed to land on a runway quite close to a parallel runway that is simultaneously departing an aircraft, despite the possibility of this resulting in a head-on collision if one or both aircraft drift off track

There can't be a head-on.  Unstated in that requirement is that both runways are used in the same direction.  If the arriving aircraft has perform a missed approach, it has procedures to direct it away from the other runway.  Also, the departing aircraft will be advised.  Under normal circumstances where the landings are executed, there's no overlap of tracks in the air, only on the runways.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Will I not be able to descend with an airfoil that stalls at -0.5°?
This stats says this airfoil stalls at -0.5°,does this means if I have this airfoil as a wing ,I won't even be able to  have a descent aoa?
!

The stall angle entry doesn’t make sense - stall should occur just above the angle for max CL which is given as 12.5°. Max L/D (best glide, not stall) is at -0.5°.

Got to be a typo. You can't get to Max Cl at 12.5deg if the wing stalls at -0.5deg

Don't believe everything that a computer tells you.
This airfoil is listed in the airfoiltools site, and I took the liberty to copy its XFOIL results here:

I selected the Reynolds numbers 500,000 and 1,000,000, which should be at the low end for a man-carrying aircraft. As you can see, the stall angle of attack is at 15° and the maximum lift coefficient is between 1.7 and 1.75. To achieve a lift coefficient of 1.89 with that airfoil will need a much higher Reynolds number, but max. L/D is already around 120.


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What does leading edge radius mean?

What does leading edge radius mean amongst these parameters of a st cyr 24 airfoil?

"st cyr 24"  Please add more description for that.

The radius of curvature at the leading edge?

@CrossRoads: See this question for an explanation. Here it is on airfoiltools.

Ok, so "st cyr 24" is an airfoil shape then.  I didn't make that connection.

The radius of the leading edge as a percentage of chord.


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Do negative angles-of-attack create lift?

(wikimedia.org) A typical curve showing section lift coefficient versus angle of attack for a cambered airfoil.
Based on the graph above, do negative angle-of-attack values create lift? How?

It's not really clear what you are asking. Is your first question just asking how to read the graph? I'm wondering because the graph clearly shows that there's lift generated with angles of attack between (just over) -5° and +25°. Thus, your first question appears to be about just reading the graph, but your second question, "how?", implies that you are able to read the graph and want an explanation as to why the lift is produced at a negative angle of attack (thus, implying that you don't actually need an answer to your first question).

@Makyen His first question asks whether the graph is correct or is an error.

@Pertinax I guess it's possible that's what was intended, but not how I read the question. If it was what was intended, wouldn't it be asked something like "The above graph shows that the airfoil produces lift at some small negative angles-of-attack. Can that really happen? If so, how?" As I read the question, it's asking for people to read the graph and state what the graph says wrt. negative angels-of-attack creating lift. Then, separately, how that happens.

You might be interested in my question Can a small GA plane maintain level flight with a significant nose-down attitude?

A symmetric airfoil will generate no lift at no angle of attack, and negative lift at a negative angle of attack. However, cambered airfoils are curved such that they will generate lift at small negative angles of attack.

Actually cambered airfoils generate lift at many angles of attack. At 0 AoA the graph shows how much lift comes only from the camber. It also shows how much negative AoA is needed to overcome the lift from the camber. At -5 AoA the two lift forces are in balance, according to this graph.

The graphic above only shows the positive $C_L$ portion of the complete curve. The line (of course!) continues below the zero $C_L$ line.
Also, the $C_L$ curve for any airfoil, symmetrical or not, must cross the $C_L$ = 0 line at some angle of attack. Whether the airfoil is symmetrical or not simply determines what that zero $C_L$ intersection AOA will be. In fact, for asymmetrical airfoils, it is only a matter of convention as to how to define Angle of Attack. Yes, it is the angle that the Airfoil makes with the relative wind (or the flight path through the air), but how the "Airfoil" is defined can vary. In some cases it is defined as the longest chord line through the airfoil from leading to trailing edge, in some cases it is defined by the bottom of the airfoil, etc.
As to the question "How?", all Aerodynamic forces are generated by the pressure of the air pushing on the surface of the airfoil. At each point on the surface of anything in a fluid, the fluid pushes on the surface, normal (perpendicular) to the surface, with whatever pressure exists at that point. What we call Lift is just an abstraction we create to help visualize and do aerodynamics calculations. It is the component of the sum of all those tiny forces added up (actually, integrated vectorially) which is perpendicular to the flight path of the aircraft. Lift is created because when you incline any airfoil, in any direction, the actual normal pressure at each point on the surface changes*, all over the surface of the airfoil, and the total vector sum of all the forces (on one side versus the other) is no longer balanced.

Why does the pressure change from one point to another? When subsonic, fluid flows are incompressible. That means that the density, (and therefore the total pressure they exert), must remain constant. So when a subsonic fluid flows, relative to a surface, since total pressure is constant, (that's the basis of the Bernoulli principle! - See Incompresible flow) and the pressure parallel to the surface (parallel to the local flow), increases, (It must since it's moving!), the pressure normal or perpindicular to the flow decreases correspondingly, to keep the total the same. And it's the normal pressure that's pushing on the surface. Basically, there is only so much energy, and it must be divided, (vectorially) between the parallel flow (dynamic) pressure and the normal pressure.

The first couple of paragraphs look perfect to me, but the part at the end where the "parallel pressure" must increase and therefore the "perpendicular pressure" must decrease (to keep the total pressure constant) ... perhaps you should think about that a little more. Or talk with someone who knows something about fluid dynamics. I don't think that part is correct.

Unfortunately, when most folks discuss Bernoulli's principal, and use the word pressure, they are really referring only to the normal pressure or pressure measured across the flow. It is obvious, that if you mount a static port at the nose of the aircraft, pointed forward, it will read higher! It is now a pitot tube, and measures dynamic pressure, not static.  When I use the phrase total pressure, I mean the vector sum of the static (or normal) pressure, and the dynamic pressure. This is also referred to as stagnation pressure, and is what must remain constant in incompressible flow.

Perhaps you should be more precise in your language, for example say dynamic pressure when you mean dynamic pressure, and stagnation pressure when you mean stagnation pressure. For many people, those are also explained by Bernoulli's principle, but if you call them out by those names at least people will know what you mean.

Air in gas phase is always compressible. The compressibility effects on fluid flow are negligible at low subsonic speeds (Mach<0.3), but saying that the fluid is incompressible is incorrect and confusing.

Sorry, Wrong. Subsonic flow is not compressible. If you try to compress it from one side, the gas on the other side just moves out of the way. Indeed, this is the main distinction between subsonic and supersonic flow. The only way to compress subsonic gas is to constrain it in a closed container, which is of course a completely different situation.

Indeed, look up any text on Incompressible flow. https://en.wikipedia.org/wiki/Incompressible_flow


text:
Is airfoil thickness proportional to drag coefficient?
Does increase in the thickness of a camber airfoil mean more drag will be produced?
Why do slow flying planes have thick airfoils when it will just slow them down even more?

There are several reasons for using thick$^*$ airfoils.

Slow planes need high lift coefficients in order to fly slow. The increased drag coefficient is the prize to pay for a higher $C_{L,max}$. Take a look at this diagram:

Source: H. Schlichting, E. Truckenbrodt, Aerodynamik des Flugzeuges. Colors added.
At low Reynolds-Numbers the maximum lift coefficient is achieved by the thickest airfoil (18%). At slightly larger Re-Numbers the 12% thickness airfoil gains the upper hand. However, the thinnest airfoil (9%) has on average about 20% less maximum lift capability than the thicker ones.
The following diagram shows polars of symmetric airfoils with different thicknesses.

Source: B. Rögner, Flugwissen. I couldn't find the original source Rögner took the diagram from, if anybody knows it feel free to comment.
The quality of image is bad, but you can recognize both the higher $C_{L,max}$ and drag coefficient of thick airfoils.
The drag coefficient depends on many factors and, as shown in the formula given in Peter's answer, is not proportional to the thickness. The drag coefficient's thickness dependency however, can be assumed to be proportional to the thickness: the $\delta^4$ term is one order of magnitude smaller than the term multiplied with $\delta$.
$$\frac{c_{D,\text{normal airfoil}}}{c_f}=2+4\cdot\delta+120\cdot\delta^4$$
$$\frac{c_{D,\text{laminar airfoil}}}{c_f}=2+2.4\cdot\delta+140\cdot\delta^4$$
In Hoerner's Fluid Dynamic Drag, Chapter 6.A.2, you can find more formulas to approximate the drag coefficient and take other parameters into account.
A good resource for understanding the influence of airfoil parameters on their aerodynamic properties is NACA TM 824.

Stall behavior of thicker airfoils is usually more forgiving.
Another reason for using thick airfoils is that it decreases the wing's weight, as thick structures can carry better the bending loads in a wing. If a wing tank is used, thicker airfoils allow for a higher tank volume.

$^*$: I interpret thick as >12%, e.g. 15% as found in the wings of many GA airplanes.

At low Reynolds-Numbers the maximum lift coefficient is achieved by the thickest airfoil (18%) .
My R.number is 1million+,I ll be using a 12hp engine for the ultralight...that means I don't have all the thrust in the world to compensate drag,do you still advice the use of a thick airfoil? @Gypaets

@DavidTeahay At Re>10^6 I'd pick an airfoil with 12% to 15% relative thickness. Compared with a 9% thick airfoil you are increasing $c_{A,max}$ by 10-20%, while the drag increase is only a single digit percentage.

understood!....I last question.@Gypaets..."slow planes need high Cl to fly".....The only airfoil with a desirable Cl for my ultralight is a Goe 518 with thickness of 18%,should I look for another with lesser thickness?

@DavidTeahay I can't really tell without knowing more details about the aircraft and how you intend to use it. I'd tend to something thinner, but the Goe is probably a valid choice too.

Alright... .......appreciate!

Yes, and even more precisely it should be worded a bit differently: The drag coefficient is grows linearly with airfoil thickness. Airfoil thickness means that the air has to flow around the airfoil. This displacement effect causes the flow around a thick airfoil to speed up more than around an equivalent but thinner airfoil. The thicker airfoil pushes the air aside and around itself more, causing the flow to accelerate and create more friction than the slower flow around a thinner airfoil. This effect is normally approximated with an additional term in the friction drag formula which is proportional to relative thickness.
Starting from the friction coefficient along a straight wall $c_f$, this additional friction drag has been captured in an empirical formula which gave the best fit to a wealth of airfoil drag data, cambered and uncambered. This is the formula for the zero-lift drag coefficient $c_{d0}$ of an airfoil:
$$c_{d0} = c_f\cdot \left(2 + 4\cdot\delta + 120\cdot\left(\frac{1}{\sqrt{1-Ma^2}}\right)^3\cdot\delta^4 - 0.09\cdot Ma^2\right)$$
where $\delta$ is the relative thickness of your airfoil and $Ma$ the Mach number.
That slow flying aircraft use thick airfoils is not generally true. However, larger aircraft want to use thicker airfoils in their wing root in order to make the wing spar lighter. By using a wider distance between the lower and upper spar caps, smaller caps can be used for the same bending strength.
In order to maximise lift, leading and trailing edge flaps are used. Thicker airfoils make their integration easier, and they allow to carry more fuel due to the wing's higher internal volume. However, beyond 20% thickness at subsonic speed and 14% at transsonic speed thickness becomes a liability - the flow will separate too early to make thicker airfoils practical.

Insightful! as always......One question: do you advice the use of a thick airfoil in an ultralight with low speed cos of the higher Cl that thick airfoils odten have

@DavidTeahay: No. The structural advantage is low and the lift coefficient is higher for airfoils of 12%. More thickness doesn't help (look at the data in Gypaets' answer - your Reynolds number is safely above 1 million). The nose shape and especially camber are more important than thickness.

@Gypaets: There is a constant (to account for both sides of the wing) and then a linear summand which is proportional with thickness. What is not proportional about that? The fact that the line will not run through the origin? That doesn't make it non-proportional.

@DavidTeahay: Please accept Gypaets' answer. Maybe he will then stop holding a grudge about my answer.

@PeterKämpf The answer is covering the zero-lift range (question is wider). I think will be useful to include the validity of this formulay (I guess not useful getting close to transonic) and also point out that proportionality might not be there when including lifting effects.


text:
Why is the critical AoA of most flaps 40° when most airfoils stall at 15°?

A plain flap deflected 40 degrees is largely stalled (...)

Flaps stall at 40° according to an answer on Quora, why is this so when typical airfoils stall at 15°?

Related

Many small planes have flaps limited to 30 degrees. It was found that more than that would out airflow over tail and lead loss of pitch control.

Flaps don't stall, it is the whole wing they are attached to. Stall normally means flow separation, but there are flaps which are designed with flow separation in their regular operating range (split flaps, for example).
Flaps change the camber of a wing such that more or less lift is created at the same angle of attack. More lift means more suction on the upper side, so the pressure recovery over the rear part of the wing needs to be steeper. Also, the surface kink of a deflected flap causes another suction spike, followed by an equally steep pressure rise. Now you have to know that any pressure rise puts big strain on the slowed-down air in the boundary layer. Too much, and the flow will separate. Enough separation and the wing stalls.
It is easy to see that more flap deflection means higher stress on the boundary layer at the same angle of attack. Therefore, the wing's stall angle of attack decreases with increasing flap deflection (the wing reference line from which the angle of attack is measured doesn't change with flap deflection, by the way). The Quora answer you linked to is full of misconceptions - please forget that nonsense about jets of air shooting from the slots. Better read this paper by a renowned expert (A. M. O. Smith, McDonnell-Douglas) - he gets the physics of flaps right!

Flaps are located in the downwash of a wing. Their local angle of attack is not equal to their deflection angle, but equal to the deflection angle minus the angle the downwash leaves the trailing edge of the wing.


text:
How would Vy be adjusted for changes in gross weight?
For piston propeller aircraft, Vy calibrated airspeed[CAS] is equal to CAS for best Lift/drag at the plane's absolute ceiling, and at lower altitudes Vy increases above best L/D as excess power is available.
For example a Cessna 172m has a best glide(approximation of best L/D) of 80mph at a MTOW of 2300 pounds, but a Sea level Vy of 90mph.
Now if you operate at 1700 pounds the L/D would be reduced approximately to $\sqrt{17/23}\times{80}= 68.8mph$ (Using mph because the 172M poh uses mph)
It would seem reasonable that sea level Vy would also be reduced by a similar factor if achieving some optimal AOA is the goal.$\sqrt{17/23}\times{90}= 77.4mph$ for the c172m example.
However the lighter weight increases the ceiling (where best L/D and Vy are equal), and because they start diverging higher the relative speed difference between best L/D and Vy will be greater at any given lower altitude.
So do the two sides fully cancel out leaving Vy unchanged? (90mph in the 172m example) What is the correct formula to make this adjustment? (exact or reliable approximation)
Do you have a reference to empirical demonstrations in support of, or in lieu of a formula?(Something with reasonably accurate metrology, not eyeballing a typical VSI)

My primary question is only about Vy. For Vx everything implies lower speed at lower gross weight, though it would be somewhat interesting to know if a basic formula exists for a Vx adjustment. Setting aside possible complications like adequate cooling at continuous low speeds.

Vx and Vy are a combination of effects between the engine performance, the propeller performance, and the aerodynamics of the airplane.
Best L/D is just determined by the aerodynamics of the airplane.
If the prop is fixed pitch, it will behave much like the airplane.  If it is constant speed, it can be a bit more complex, but it still behaves mostly like the airplane.
We generally ignore the performance of the engine.
So, that leaves us with the aerodynamics of the aircraft.  This is the dominant factor.
Vx, Vy, and best L/D each happen at a certain lift coefficient.
For a simplified piston-prop,
Vy occurs at best $\frac{{C_L}^{3/2}}{C_D}$.  Which, for a simple parabolic drag polar occurs when $C_L=\sqrt{\frac{3\,C_{D,0}}{K}}$.
Best L/D occurs at best $\frac{C_L}{C_D}$ (obviously).  Which, for a simple parabolic drag polar occurs when $C_L=\sqrt{\frac{C_{D,0}}{K}}$.
Your error is that the point of absolute ceiling occurs at best $L/D$.  For a simplified piston propeller aircraft, we assume that the power available is constant with airspeed.  Therefore, ceiling will happen at the speed for minimum power required.
In a jet aircraft, we assume that the thrust available is constant with airspeed -- in that case, ceiling will happen at the speed for minimum drag (best L/D).
The rest of this should be familiar to you -- I typed it up before I re-read your question to focus on the issue of ceiling...
$C_L = \frac{W}{q\,S}$
So, we want to look at what happens when we change $W$ but keep $C_L$ constant.
Re-arranging for dynamic pressure
$q = \frac{W}{C_L\,S}$
We see that any increase in $W$ needs a proportional increase in $q$.  The dynamic pressure is:
$q = 0.5\,\rho\,V^2$
Substituting gives
$0.5\,\rho\,V^2 = \frac{W}{C_L\,S}$
Solving for V gives
$V = \sqrt{\frac{2\,W}{\rho\,C_L\,S}}$
So, any velocity flown at the same density altitude ($\rho$) and a fixed lift coefficient (say that for Vx, Vy, or best range) will scale with the square root of weight -- because everything else in the equation is constant.
I.e. you could write this equation twice -- once for a current condition, and a second time for a reference condition.  Then divide current by reference and cancel whatever is constant.  You will be left with this....
$\frac{V}{V_\mathrm{ref}} = \sqrt{\frac{W}{W_\mathrm{ref}}}$
(at the same density altitude)
This was done in terms of true airspeed, but it works out the same in terms of equivalent airspeed.  Just use the equation for dynamic pressure in terms of equivalent airspeed instead.
$q = 0.5\,\rho_0\,V_e^2$
This works out to
$\frac{V_e}{V_{e,\mathrm{ref}}} = \sqrt{\frac{W}{W_\mathrm{ref}}}$
Which, due to the magic of equivalent airspeed, is true for any density altitude.